how to include long text

i am working on smartforms, standard smart form BBP_PO is copied to zzlayout, i have included a new window to it, SF_PO is there in the form interface, since this is standard form they have already reterieved the values for SF_PO. if i try to use the SF_PO_IT_LONGTEXT-TDLINE it is showing an error message component does not exist. what is the problem.